Steel Material and Process for Producing the Same

ABSTRACT

A steel material of 85 to 95 H RB  exhibiting a ratio of area occupied by ferrite of 30% or higher, which steel material comprises given amounts of C, Si, Mn, S, Ti, Cr, Al and B and the balance of iron and unavoidable impurities, is subjected to induction hardening. After the hardening, the surface of the steel material exhibits an H v  value of 640 to 730. When t refers to the effective hardening depth, namely, the distance from the surface to a region exhibiting an H v  value of 392 and r refers to the radius of the steel material, the hardened layer ratio, t/r, is 0.4 or higher.

TECHNICAL FIELD

The present invention relates to a steel member (material) and a method (process) for producing the same, particularly to a steel member suitable for a drive shaft, for example, and a method for producing the same.

BACKGROUND ART

Drive shafts for engines of automobiles are generally made of a steel member. Such a steel member is required to be processed relatively easily by cutting work, etc. Meanwhile, in view of ensuring durability as the drive shafts, the steel member needs to be high in shear stress, under which the member is broken by applying torsion torque, in other words, in torsion strength. For example, JIS-S40C equivalent materials are used as the steel member to obtain the drive shafts.

In recent years, with the increased interest in environmental conservation, various studies have been made on improving automobile fuel consumption, thereby reducing the amount of an exhaust gas such as CO₂ or NOx. From this viewpoint, size reduction of an automobile component has been tested to reduce the weight.

When a smaller-sized drive shaft is produced using the same steel member, the resultant shaft generally has a lower strength, etc. Thus, there is a demand for a steel member useful for producing a smaller-sized drive shaft having a sufficient strength, etc.

In Patent Document 1, for example, a drive shaft composed of a steel member having predetermined constituent element composition ratio, surface hardness, martensite content, and hardening depth ratio is proposed to meet the demand.

Further, in Patent Document 2, a steel for induction hardening, which has ferrite crystal grain size and area ratio of ferrite structure to C component of predetermined values or less, is disclosed. According to Patent Document 2, this steel for induction hardening is suitable as a material for a drive shaft.

Patent Document 1: Japanese Laid-Open Patent Publication No. 10-036937 Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-069566 DISCLOSURE OF THE INVENTION

A general object of the present invention is to provide a steel member useful for producing a small-sized member having excellent properties.

A principal object of the present invention is to provide a method for producing the steel member.

According to an aspect of the present invention, there is provided a steel member comprising, in percent by mass, 0.47% to 0.52% of C, 0.03% to 0.15% of Si, 0.6% to 0.7% of Mn, 0.005% to 0.03% of S, 0.025% to 0.04% of Ti, 0.05% to 0.3% of Cr, 0.04% to 0.09% of Mo, 0.02% to 0.04% of Al, and 0.0005% to 0.004% of B, the balance being iron and inevitable impurities, wherein the steel member has a surface with a Vickers hardness of 640 to 730, and when the steel member has a radius r and an effective hardening depth t that is a distance between the surface and a region with a Vickers hardness of 392, a hardened layer ratio t/r of the steel member is 0.4 or more.

By controlling the hardness and the hardened layer ratio as above, the steel member can secure sufficient hardness and strength. This is because materials having high hardness generally show high strength. Further, the hardness of the steel member is not excessively high, so that the steel member is tough and is hardly brittle-fractured.

Further, by using the above components and composition ratios, the steel member can be excellent in cutting properties.

A nitrogen content of the steel member is preferably 0.01% by mass or less. In this case, BN and TiN are hardly generated. Therefore, the hardening of the member can proceed without inhibition thereof, and the member can be prevented from being reduced in the machinability and cutting properties due to excessively increased hardness.

According to another aspect of the present invention, there is provided a method for producing a steel member, wherein the steel member has a surface with a Vickers hardness of 640 to 730, and when the steel member has a radius r and an effective hardening depth t that is a distance between the surface and a region with a Vickers hardness of 392, a ratio t/r of the steel member is 0.4 or more. The method comprises subjecting a steel material to induction hardening, in which the steel material comprises, in percent by mass, 0.47% to 0.52% of C, 0.03% to 0.15% of Si, 0.6% to 0.7% of Mn, 0.005% to 0.03% of S, 0.025% to 0.04% of Ti, 0.05% to 0.3% of Cr, 0.02% to 0.04% of Al, and 0.0005% to 0.004% of B, the balance being iron and inevitable impurities, and has an occupied area ratio by ferrite of 30% or more and a B-scale Rockwell hardness of 85 to 95.

Thus, by subjecting the steel material having the above components and composition ratios to the induction hardening, the steel member excellent in hardness, strength, and cutting properties can be obtained.

A small-sized member excellent in strength can be produced using the steel member having the above properties. The steel member is excellent in the cutting properties and plastic deformability, and can be remarkably easily processed. Further, the steel member is excellent also in toughness, and thereby is hardly cracked in a processing step.

Furthermore, the steel member has high torsion strength, and thus can be suitably used as a material for a long shaft member such as a drive shaft.

As described above, by controlling the components, composition ratios, hardness, and hardened layer ratio, the steel member excellent in strength, particularly torsion strength, and excellent in cutting properties can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall, longitudinal, schematic side view of a drive shaft comprising a steel member according to an embodiment of the present invention;

FIG. 2 is a graph showing the relation between the hardness and the distance from the surface of the drive shaft of FIG. 1;

FIG. 3 is a table showing components and composition ratios of each steel member;

FIG. 4 is a table showing properties of the steel members of FIG. 3; and

FIG. 5 is a graph showing the relation between the shear stress and the hardened layer ratio r/t of the steel member of each Example.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the steel member and the producing method of the present invention will be described in detail below with reference to attached drawings.

FIG. 1 is an overall, longitudinal, schematic side view of a drive shaft 10. The drive shaft 10 is a long solid body, and a measurement site represented by the referential mark 12 in FIG. 1 has a diameter of 28 mm.

The drive shaft 10 comprises a steel member, which contains C, St, Mn, S, Ti, Cr, Mo, Al, and B in addition to iron and inevitable impurities.

C is a component for ensuring the strength and hardness, etc. of the drive shaft 10 after hardening and tempering treatments. The composition ratio thereof is controlled to 0.47% to 0.52% (by mass, also the following composition ratio values are in percent by mass). When the composition ratio is less than 0.47%, it is difficult to ensure the hardness of the steel member. On the other hand, when it is more than 0.52%, the hardness is excessively increased, whereby it is difficult to process the steel member by plastic deformation, cutting, etc. and the drive shaft 10 is likely to be brittle-fractured with a lowered strength. Particularly, the cold working deformability of the steel member is reduced.

Si is an element useful for deoxidation of the drive shaft 10, and the Si composition ratio of the drive shaft 10 is controlled to 0.03% to 0.15%. When the composition ratio is less than 0.03%, Si shows a poor deoxidation effect. On the other hand, when it is more than 0.15%, the hardness of the steel member is excessively increased, whereby it is difficult to process the steel member by plastic deformation, cutting, etc. Particularly, the cold working deformability of the steel member is reduced.

Mn acts to increase the induction hardening properties of the drive shaft 10. Thus, owing to the presence of Mn, the drive shaft 10 has a significantly higher hardness after an induction hardening, as compared with before the hardening. The Mn composition ratio is controlled to 0.6% to 0.7%, to reliably obtain this effect. When the composition ratio is more than 0.7%, the hardness of the steel member is excessively increased, whereby it is difficult to process the steel member by plastic deformation, cutting, etc. Particularly, the cold working deformability of the steel member is reduced.

S is a component that forms MnS together with Mn in the structure of the drive shaft 10, thereby increasing the cutting properties of the drive shaft 10. The S composition ratio is controlled to 0.005% to 0.03%. When the composition ratio is less than 0.005%, it is difficult to increase the cutting properties. When it is more than 0.03%, particularly the cold working deformability of the steel member is reduced.

Ti acts to capture free N in the drive shaft 10. When the free N is captured, the effect of B addition, to be hereinafter described, is increased. The Ti composition ratio is controlled to 0.025% to 0.04%. When the composition ratio is less than 0.025%, Ti shows a poor free N capturing effect. On the other hand, when it is more than 0.04%, the steel member contains excessive Ti, so that the cutting properties and the cold working deformability of the member are reduced.

Cr is a component for improving the hardening properties of the drive shaft 10. In other words, owing to the presence of Cr, the drive shaft 10 has a significantly higher hardness after a hardening treatment. The Cr composition ratio is controlled to 0.05% to 0.3%. When the composition ratio is less than 0.05%, this effect is hardly obtained. On the other hand, when it is more than 0.3%, Cr is concentrated in cementite and prevents solid dissolution of carbon in the steel member during the hardening treatment.

Mo is an element useful for increasing the grain boundary strength of the drive shaft 10 after the hardening treatment, particularly the torsion strength. The Mo composition ratio is controlled to 0.04% to 0.09%. When the composition ratio is less than 0.04%, the grain boundary strength is hardly increased. On the other hand, when it is more than 0.09%, the hardness of the steel member is excessively increased, whereby it is difficult to process the steel member by plastic deformation, cutting, etc. Particularly, the cold working deformability of the steel member is reduced.

B is a component for increasing the grain boundary strength, and for increasing the hardening properties of the drive shaft 10. This effect is reduced when the steel member contains excessive N. This is because B forms BN together with the excessive N. Thus, as described above, N is captured by the predetermined amount of Ti to secure this effect.

The B composition ratio is controlled to 0.0005% to 0.004% (5 to 40 ppm). When the composition ratio is less than 5 ppm, B shows a poor effect of increasing the grain boundary strength. On the other hand, when it is more than 40 ppm, the hardening properties are reduced.

Al is a component that contributes to deoxidation in the same manner as Si. When the Al composition ratio is less than 0.02%, Al shows a poor deoxidation effect. On the other hand, when the ratio is excessively large, oxide impurities such as Al₂O₃ are increased, so that the fatigue properties and plastic deformability of the steel member are deteriorated. Thus, the upper limit of the Al composition ratio is 0.04%.

In a case where the drive shaft 10 contains an excessive amount of the free N, the N is bonded to B to form BN, so that the hardening properties, etc. are reduced as described above. Further, when the N is bonded to Ti to form TiN, the hardness of the steel member is excessively increased, so that the processing of the member is difficult, and the toughness is reduced. In this embodiment, the N (nitrogen) composition ratio of the drive shaft 10 is controlled to 0.01% or less to avoid such circumstances.

The occupied area ratio by ferrite observed in the structure of the drive shaft 10 is 30% or more. Thus, the ratio of the area occupied by the ferrites to the entire visible area (100%) is 30% or more. When the ferrite area ratio is within the range, the drive shaft 10 is excellent in toughness after the hardening.

The surface hardness of the drive shaft 10 is such that the B-scale Rockwell hardness (H_(RB)) is 85 to 95. When the surface hardness is within the range, the strength of the drive shaft 10 is maintained after the hardening and tempering treatments.

For example, the drive shaft 10 may be produced by subjecting a cylindrical workpiece, comprising the steel member having the above components and composition ratios, to turning or component rolling processing, etc.

The drive shaft 10 is then subjected to the hardening and tempering treatments.

In this embodiment, an induction hardening method using high-frequency heating is adopted in the hardening treatment. Thus, first the surface of the drive shaft 10 is rapidly heated by a high-frequency induction current, and then the surface is rapidly cooled by spraying a cooling liquid. The hardening conditions may be such that the induction current frequency is about 1 to 40 kHz, the induction current output is about 50 to 100 kW, and the heating time is 1 to 5 seconds.

Then, the drive shaft 10 is subjected to the tempering treatment within a temperature range of 150° to 200° C. By the treatment, the residual stress of the drive shaft 10 can be removed, and the secular change or cracking in the drive shaft 10 can be reduced.

The surface of the drive shaft 10, hardened and tempered as described above, has a Vickers hardness (H_(v)) of 640 to 730. Such a steel member having high hardness generally show high strength, and the steel member is not insufficient in toughness if the hardness is within the above range.

In the drive shaft 10, the hardness is reduced in the radial direction from the surface to the inside as shown in FIG. 2. In this embodiment, the hardness of the drive shaft 10 is measured from the surface, and the distance (depth) between the surface and a region with an H_(v) of 392 is referred to as the effective hardened layer depth t. For example, since the measurement site 12 has a diameter of 28 mm as described above, the value of 14 mm on the horizontal axis of the graph shown in FIG. 2 is at the radial direction center of the measurement site 12.

When r represents the radius of the drive shaft 10, the ratio of the effective hardened layer depth t to the radius r (the hardened layer ratio t/r) is 0.4 or more. When the ratio is less than 0.4, the thickness of the effective hardened layer is insufficient, whereby the drive shaft 10 is low in the torsion strength.

Thus, by controlling the surface H_(v) to 640 to 730 and by controlling the hardened layer ratio t/r to 0.4 or more, the drive shaft 10 excellent in the strength and toughness can be obtained.

Though the drive shaft 10 is described as an example of the steel member in the above embodiment, the final product of the steel member is not particularly limited thereto and may be another one such as an outer member of a constant-velocity Joint.

EXAMPLE 1

Ingots of steel members having components and composition ratios shown in FIG. 3 were prepared respectively using a vacuum melting furnace. Each ingot was heated at 950° C. and hot-forged to produce a cylindrical workpiece having a diameter of 27 mm. The temperature at the end of the forging was between the Ac3 point and 880° C.

Each cylindrical workpiece was observed using an optical microscope, to obtain the ferrite area ratio. Meanwhile, the H_(RB) of the cylindrical workpiece, at the depth of 7 to 8 mm from the surface, was measured.

A small cylindrical workpiece having a diameter of 14 mm and a length of 21 mm was cut from the cylindrical workpiece. Then, the small cylindrical workpiece was subjected to an upset forming at a temperature within a cold working temperature range, and the upset ratio to crack initiation was obtained.

Further, the cylindrical workpiece was subjected to a cutting test using ST20E (trade name, a cutting tool manufactured by Sumitomo Electric Industries, Ltd.) under conditions of a cutting speed of 150 m/minute, a cutting depth of 0.5 mm, and a feed speed of 0.2 mm/rev, and the abrasion loss of ST20E was measured after 12 minutes.

Further, a drive shaft 10 having a shape shown in FIG. 1 was produced from each cylindrical workpiece, and the drive shaft 10 was subjected to induction hardening and tempering treatments under various conditions to change the effective hardened layer depth t and the hardened layer ratio t/r. Then, each drive shaft 10 was subjected to a static torsion test.

The same experiment was carried out using each of the steel members having the composition and component ratios shown in FIG. 3 for comparison.

The results are shown in FIG. 4. It is clear from FIG. 4 that the steel members having controlled components, composition ratios, ferrite area ratios, and hardnesses have excellent deformability, good cutting properties, and high torsion strength.

The relation between the shear stress and the hardened layer ratio t/r of the steel member of each Example is shown in the graph of FIG. 5. As the shear stress is larger, the static torsion strength is higher. It should be noted that the dash line in FIG. 5 represents the relation between the shear stress and the hardened layer ratio of the S40C equivalent material.

It is clear from FIG. 5 that each steel member of Examples has a static torsion strength higher than that of the S40C equivalent material. 

1. A steel member comprising, in percent by mass, 0.47% to 0.52% of C, 0.03% to 0.15% of Si, 0.6% to 0.7% of Mn, 0.005% to 0.03% of S, 0.025% to 0.04% of Ti, 0.05% to 0.3% of Cr, 0.04% to 0.09% of Mo, 0.02% to 0.04% of Al, and 0.0005% to 0.004% of B, the balance being iron and inevitable impurities, wherein said steel member has a surface with a Vickers hardness of 640 to 730, and when said steel member has a radius r and an effective hardening depth t that is a distance between said surface and a region with a Vickers hardness of 392, a hardened layer ratio t/r of said steel member is 0.4 or more.
 2. A steel member according to claim 1, wherein a nitrogen content of said steel member is 0.01% by mass or less.
 3. A steel member according to claim 1, wherein said steel member is a drive shaft.
 4. A method for producing a steel member, wherein said steel member has a surface with a Vickers hardness of 640 to 730, and when said steel member has a radius r and an effective hardening depth t that is a distance between said surface and a region with a Vickers hardness of 392, a ratio t/r of said steel member is 0.4 or more, said method comprising subjecting a steel material to induction hardening, wherein said steel material comprises, in percent by mass, 0.47% to 0.52% of C, 0.03% to 0.15% of Si, 0.6% to 0.7% of Mn, 0.005% to 0.03% of S, 0.025% to 0.04% of Ti, 0.05% to 0.3% of Cr, 0.04% to 0.09% of Mo, 0.02% to 0.04% of Al, and 0.0005% to 0.004% of B, the balance being iron and inevitable impurities, and has an occupied area ratio by ferrite of 30% or more and a B-scale Rockwell hardness of 85 to
 95. 5. A method according to claim 4, wherein said steel material is formed into a drive shaft and then subjected to said induction hardening. 